This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-049258, filed Feb. 23, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an X-ray CT apparatus capable of obtaining a tomographic image of an object of inspection in a short time.
2. Description of the Related Art
An X-ray CT (Computed Tomography) apparatus comprises an X-ray tube and a detector that are located on either side of an object of inspection. The X-ray tube emits the X rays toward the object of inspection, and the detector detects the X rays emitted from the X-ray tube. Based on the difference in the rate of X-ray absorption between parts in the object of inspection, the X-ray CT apparatus analyzes, by using a computer, transmitted X-ray data that the detector detects each time when X rays are applied to the object of inspection in many directions, and obtains a tomographic image of the object of inspection.
There is an X-ray CT apparatus of an Rxe2x80x94R (rotatexe2x80x94rotate) type in which the X-ray tube and detector rotate around the object of inspection while maintaining their relative positions. In the case where the X-ray CT apparatus of this type is used for diagnosis in the medical field, however, it involves mechanical movement such that the X-ray tube and the detector rotate around a patient, the object of inspection, so that the taking time (scanning time), which is necessary to obtain one tomographic image, is long.
There is also an X-ray CT apparatus of another type. This X-ray CT apparatus comprises an electron gun, a magnetic field coil, and a target ring instead of the X-ray tube. The electron gun is located on the central axis of the target ring. An electron beam shot from the electron gun toward the center of the target ring is bent by the magnetic field coil, whereupon it hits the target ring. When the target ring is hit by the electron beam, it emits an X-ray toward an object of inspection located inside the target ring. The X-ray CT apparatus changes the position for the impact of the electron beam in the circumferential direction of the target ring by changing the direction of the magnetic field that the magnetic field coil generates. By doing this, the X-ray CT apparatus applies X rays to the object of inspection in many directions, and obtains a tomographic image in accordance with the resulting transmitted X-ray data. In the X-ray CT apparatus of this type, the X-ray source is moved electrically, so that the taking time can be made shorter than in the case of the X-ray CT apparatus of the Rxe2x80x94R type. However, there is a limit to shorten the time for acquiring a tomographic image, because the X-ray source is only one in number at the same time and a detector takes time to detect the necessary dose of X rays.
In the industrial field, there is proposed an X-ray CT apparatus for observing the movement of air bubbles in two-phase flow, for example. As an example of this X-ray CT apparatus, there is an X-ray CT apparatus comprising a large number of X-ray sources as described in Jpn. Pat. Appln. KOKAI Publication No. 9-248300, 10-75944, or 10-295682.
The X-ray CT apparatuses described in Jpn. Pat. Appln. KOKAI Publications Nos. 9-248300 and 10-75944 use a large number of X-ray tubes as X-ray sources. In the case described in Jpn. Pat. Appln. KOKAI Publication No. 10-75944, X rays are simultaneously applied from two or more X-ray tubes on condition that regions on the detector upon which X rays are incident never overlap one another.
The X-ray CT apparatus described in Jpn. Pat. Appln. KOKAI Publication No. 10-295682 comprises a vacuum chamber, a large number of X-ray sources, and a detector. The vacuum chamber is in the shape of a ring that surrounds the object of inspection. The numerous X-ray sources are arranged in a circumferential direction in the vacuum chamber, and emit X rays in fan beam that crosses the object of inspection toward the object of inspection. The detector is in the shape of a ring that surrounds the object of inspection in a position on the inner peripheral side of the vacuum chamber, and serves to detect the X rays that are emitted from the X-ray sources and passed through the object of inspection. The X-ray sources are actuated one after another in the order of arrangement and emit the fan-shaped X rays toward the object of inspection. The emitted X rays are passed through the object of inspection and detected by the detector on the opposite side. A tomographic image can be obtained in a short time by quickly switching signals that serve to actuate the X-ray sources.
Accordingly, application of industrial X-ray CT apparatuses to an examination in the medical field is under investigation. When a patient is performed diagnosis by using an industrial X-ray CT apparatus, the taking time for the acquisition of a tomographic image can be shortened considerably, so that improvement of the efficiency of diagnosis can be expected. Since the industrial X-ray CT apparatus can acquire a tomographic image in a short time, moreover, an image can be obtained corresponding to change that is occurred in a short time.
In the case where X rays are generated by utilizing an electron beam, X rays are emitted in all directions from the spot that is hit by the electron beam. However, no consideration is given to the spread angle of the fan-beams that are emitted from the X-ray sources of the industrial X-ray CT apparatus. Thus, the object of inspection is irradiated with unnecessary X rays that are not detected by means of the detector.
Further, no consideration is given to the spread angle of X rays in the industrial X-ray CT apparatus. When the X-ray CT apparatus irradiates the fan-beams from three or more X-ray sources to the object of inspection at a time, the X rays inevitably interfere with one another in some regions on the detector and a high-precision tomographic image cannot be obtained. Therefore, the X rays must be emitted from opposite positions at 180xc2x0 from each other in order to apply the X rays without interference. In the case where the X-ray CT apparatus irradiates the fan-beams from two X-ray sources to the object of inspection at a time, the two X-ray sources must be switched so that they always maintain their symmetrical positions with respect to the object of inspection. The time for taking the acquisition of one tomographic image is restricted by the time that is necessary to switch each X-ray source around the object of inspection by a half turn.
The X-ray CT apparatus for diagnosis in the medical field must apply the X rays to the object of inspection in as many directions as possible to acquire transmitted X-ray data in order to obtain a fine tomographic image. In the case where the industrial X-ray CT apparatus is applied to the medical field, therefore, the X-ray sources must be increased in number.
When the directions in which X rays are applied is increased, however, the dose of X rays applied to the patient inevitably increases, and the taking time is prolonged, so that the load on the patient increases.
The purpose of the present invention is to provide an X-ray CT apparatus designed so that the dose of X rays applied to the object of inspection can be reduced, and that the time, which is necessary to acquire a tomographic image of the object of inspection, can be shortened.
An X-ray CT apparatus according to one embodiment of the present invention is designed to reduce the dose of X rays applied to an object of inspection and to shorten the time that is necessary to acquire a tomographic image of the object of inspection.
An X-ray CT apparatus according to one embodiment of the present invention includes a large number of X-ray sources, a detector, and a collimator. The X-ray sources are arranged around the object of inspection. The detector detects X rays emitted from the X-ray sources. The collimators are located between the X-ray sources and the object of inspection, thereby restricting those X rays that, among the X rays emitted from the X-ray sources, are not applied to the detection surface of the detector.
An X-ray CT apparatus according to another embodiment of the present invention includes a main body, a large number of X-ray sources, a vacuum chamber, a collimator, a detector, a bed, and a beam limiter. The main body has a hole in which an object of inspection is located. The X-ray sources are concentrically arranged around the hole. The vacuum chamber is in the form of a ring surrounding the hole and holds the X-ray sources. The collimator is mounted along the inner peripheral wall of the vacuum chamber and has through holes corresponding to the individual X-ray sources. The detector includes a large number of detection elements for detecting the X rays emitted from the X-ray sources. The detection elements are arranged densely in the shape of a cylinder having the same central axis with a concentric circle composed of the X-ray sources, with the detection surface thereof facing toward the central axis. The bed has a slide mechanism and a lift device and serves to position the object of inspection in the hole of the main body. The beam limiter is located between the X-ray sources and the object of inspection and serves to restrict the spread of the X rays in the direction along the central axis of the concentric circle composed of the X-ray sources within the width of the detector in the direction along the central axis.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.